Of course the tokens can be obfuscated, but at some point the tokens must be used in plain-text to sign the OAuth request, and on a device like an iPhone where complete control of execution flow and full address space access is possible (via kernel exploits / jailbreak and a debugger), it's literally impossible to prevent this attack.
Issuing unique per-device device tokens would be an interesting approach, but then a trusted side-band would be necessary to issue the tokens to the device, and a lot of the point of OAuth is moot.
OAuth is still quite useful for user authentication (as to steal per-user tokens requires access to the user's device anyway).
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